The present invention relates to an improved process for deasphalting a residua feedstock by use of a short vapor residence time process unit comprised of a horizontal moving bed of fluidized and/or stirred hot particles. The vapor phase product stream from said process unit is passed to a soaker drum where a high boiling fraction is separated and recycled to the process unit after undergoing reactions causing molecular weight growth. This reactive recycle using the soaker drum results in substantially improved qualities of the liquid products compared with what is achieved by once-through residua deasphalting process alternatives.
In a typical refinery, crude oils are subjected to atmospheric distillation to separate lighter materials such as gas oils, kerosenes, gasolines, straight run naphtha, etc. from the heavier materials. The residue from the atmospheric distillation step is then distilled at a pressure below atmospheric pressure. This later distillation step produces a vacuum gas oil distillate and a vacuum reduced residual oil that often contains relatively high levels of asphaltene molecules. These asphaltene molecules usually contain most of the Conradson Carbon residue and metal components of the resid. They also contain relatively high levels of heteroatoms, such as sulfur and nitrogen. Such feeds have little commercial value, primarily because they cannot be used as a fuel oil because of ever stricter environmental regulations. They also have little value as feedstocks for refinery processes, such as fluid catalytic cracking, because they produce excessive amounts of gas and coke. Also, their high metals content leads to catalyst deactivation. Thus, there is a great need in petroleum refining to upgrade residual feeds to more valuable cleaner and lighter feeds.
There are a number of techniques used for recovering the lighter components from various asphaltic petroleum residual feeds. Many such processes involve the extraction of the lighter components with a deasphalting solvent such as propane, and thereafter separating and recovering the lighter components from the solvent. In U.S. Pat. No. 2,950,244, a process for the extraction of petroleum residue containing asphalt is disclosed. The solvent utilized is a liquefied normally gaseous solvent, such a propane, which is maintained at a temperature between about 100xc2x0 F. and 200xc2x0 F. and at a pressure sufficient to maintain the solvent in a liquid phase.
Variations of the deasphalting process using propane, or similar short chain aliphatics as solvents, are taught in U.S. Pat. No. 2,669,538 to Yuraski et al.; U.S. Pat. No. 3,516,928 to King et al. issued Jun. 23, 1970; U.S. Pat. No. 4,017,383 to Beavon, issued Apr. 12, 1977; and U.S. Pat. No. 4,201,660 to Szosel, issued May 6, 1980. King et al. additionally suggest that carbon dioxide and ammonia, under certain circumstances are equivalent solvents to the lower alkanes, alkenes, and their halogenated derivatives.
While propane is often used in conventional solvent deasphalting operations, other solvents have been suggested. For example, in U.S. Pat. No. 4,054,512, an asphalt-containing mineral oil is deasphalted by contacting the oil with liquid hydrogen sulfide. The use of liquid neopentane, at a temperature between 0xc2x0 F. and 250xc2x0 F., taught in U.S. Pat. No. 3,334,043. Also, in U.S. Pat. No. 2,337,448, heavy residual oil is deasphalted by a solvent selected from the group consisting of ethane, ethylene, propane, propylene, butane, butylene, isobutane, and mixtures thereof.
U.S. Pat. No. 4,191,639 to Audeh et al teaches a process wherein hydrocarbon oils, such as residual petroleum oils, are deasphalted and demetallized by contact with a liquid mixture of at least two of the components selected from hydrogen sulfide, carbon dioxide, and propane.
Also, U.S. Pat. No. 5,714,056 teaches a process for deasphalting residua in a short vapor contact time thermal process unit comprised of a horizontal moving bed of fluidized hot particles. This is a once through process whereby the removal of feed contaminants is limited to what can be achieved in a single pass. There is no suggestion of separating a high boiling fraction from the vapor product fraction and recycling it to the reaction zone.
While solvent deasphalting has met with commercial success, there is nevertheless a continuing need in the art for deasphalting processes which result in higher liquid yields and improved liquid product quality than solvent deasphalting. There is also a need in the art for improved processes capable of deasphalting an asphalt-containing residual feedstock without the use of a solvent.
In accordance with the present invention there is provided a process for deasphalting an asphalt-containing feedstock in a deasphalting process unit comprised of:
(i) a heating zone wherein solids containing carbonaceous deposits are received from a stripping zone and heated in the presence of a heating gas which may contain oxygen for partial combustion purposes;
(ii) a short vapor residence time reaction zone containing a horizontal moving bed of fluidized and/or stirred hot solids recycled from the heating zone and feed, which reaction zone is operated at a temperature from about 450xc2x0 C. to about 700xc2x0 C. and operated under conditions such that the solids residence time and the vapor residence time are independently controlled, which vapor residence time is less than about 5 seconds, and which solids residence is from about 5 to about 60 seconds; and
(iii) a stripping zone through which solids having carbonaceous deposits thereon are passed from the reaction zone and wherein lower boiling additional hydrocarbon and volatiles are recovered with a stripping gas; which process comprises:
(a) feeding the residua feedstock to the short vapor residence time reaction zone wherein it contacts the hot solids thereby resulting in high Conradson Carbon components and metal-containing components being deposited onto said hot solids, and a vaporized fraction;
(b) separating the vaporized fraction from the solids; and
(c) passing the solids to said stripping zone where they are contacted with a stripping gas, thereby removing volatile components therefrom;
(d) passing the stripped solids to a heating zone where they are heated to an effective temperature that will maintain the operating temperature of the reaction zone;
(e) recycling hot solids from the heating zone to the reaction zone where they are contacted with fresh feedstock;
(f) passing the vaporized fraction from step (b) above to a soaker drum where it is quenched to produce a vapor fraction boiling less than about 450-600xc2x0 C. and a high boiling fraction condensate having an initial boiling point in the range of about 450-600xc2x0 C.;
(g) providing sufficient residence time and reactor severity in the soaker drum to permit molecular weight growth reactions to occur; (h) recycling said high boiling fraction to the short vapor contact time reaction zone; and
(i) recovering the vapor fraction having a lower concentration of contaminants from step (h).
In a preferred embodiment of the present invention, steam, C4 minus gas, or both, is injected into the soaker drum to maintain the solids in suspension and to strip out lower boiling range products.
In another preferred embodiment of the present invention the soaker drum is operated at increased pressure and temperature to reduce reaction time and therefore the soaker drum size.
In a preferred embodiment of the present invention, the particles of the short contact time reaction zone are fluidized and/or stirred with the aid of a mechanical means.
In another preferred embodiment of the present invention, steam, C4 minus gas, or both, is injected into the vaporized fraction upstream of the soaker drum to reduce the partial pressure of the C5 plus hydrocarbon to condense the high boiling fraction as per step (f) at a temperature which is lower than its initial boiling point of about 450-600xc2x0 C.
In another preferred embodiment of the present invention polymerization initiators are present in the soaker drum to increase reaction rates.
In yet another preferred embodiment of the present invention the soaker drum can include a mechanical mixing device providing the advantages of self-cleaning to minimize coke deposits and to achieve substantially plug flow reaction conditions of the liquid phase.